Bloom's syndrome (BS) is an autosomal recessive disorder characterized clinically by growth deficiency, immunodeficiency, reduced fertility, and an enormous predisposition to cancer of a wide variety of types and sites. Although the gene is believed to be involved in some nuclear way with maintenance of genomic stability, the primary defect in BS has not been identified. The purpose of the work proposed is to map and clone the Bloom's syndrome locus, BLM. this goal will be accomplished by the following plan. 1.Sub-localize the BLM locus on chromosome No. 15 by linkage analysis using consanguinity mapping. 2. Isolate highly polymorphic DNA markers from the region defined by the linkage (the BLM region) by cloning DNA segments from radiation-reduced hybrids and microdissected chromosomes. 3. Refine the map location of these markers in the BLM region by consanguinity mapping, linkage-disequilibrium mapping, and long-range restriction mapping. 4. Isolate the BLM region using YACs (yeast artificial chromosomes) identified by the markers closest to BLM as defined in 3. 5. Identify candidates for BLM, employing DNA-mediated transformation to test their ability to correct the high-SCE phenotype of BS cells. To be tested will be (a) DNA from whole YACs and (b) cDNA identified using YACs as hybridization probes. 6. Confirm a candidate gene by identification of mutation(s) at the molecular level in the BLM locus in individuals with Bloom's syndrome. The fulfillment of the goal will provide fundamental knowledge about a key aspect of DNA replication and repair. this knowledge will lead to understanding the activity of the wild type BLM gene, the role of the defective gene product from the mutated BLM locus, and how the mutated gene is involved in the clinical phenotype. With the current advances in the field of human genetics on the "chromosome-breakage syndromes" that as a group appear related to DNA replication and repair - and of which BS is the prototype - the sum total of knowledge may yield an integrated and in time complete view of this complex process, and in a broader sense, the way somatic cells maintain genetic stability.